This invention relates to pressurized plastic containers that have enhanced barrier performance and methods to provide said containers and to the coatings. The enhanced barrier performance is obtained by application of inorganic coatings to the external surface of the container. The coatings exhibit enhanced adhesion relative to prior art coatings. In addition, this invention also relates to recycling of coated plastic containers and the packaging of beverages in said container.
Plastic containers currently comprise a large and growing segment of the food and beverage industry. Plastic containers offer a number of advantages over traditional metal and glass containers. They are lightweight, inexpensive, nonbreakable, transparent and easily manufactured and handled. However, plastic containers have at least one significant drawback that has limited their universal acceptance, especially in the more demanding food applications. That drawback is that all plastic containers are more or less permeable to water, oxygen, carbon dioxide, and other gases and vapors. In a number of applications, the permeation rates of affordable plastics are great enough to significantly limit the shelf-life of the contained food or beverage, or prevent the use of plastic containers altogether.
It has been recognized for some time that a container structure that combines the best features of plastic containers and more traditional containers could be obtained by applying a glass-like or metal-like layer to a plastic container, and metallized plastic containers. For example, metallized potato chip bags have been commercially available for some time. However, in a number of applications, the clarity of the package is of significant importance, and for those applications metallized coatings are not acceptable. Obtaining durable glass-like coatings on plastic containers without changing the appearance of the container has proven to be much more difficult.
A number of processes have been developed for the purpose of applying glass-like coatings onto plastic films, where the films are then subsequently formed into flexible plastic containers. However, relatively few processes have been developed that allow the application of a glass-like coating onto a preformed, relatively rigid plastic container such as the PET bottles commonly used in the U.S. for carbonated beverages, and heretofore no process has been developed that allows the application of a glass-like coating onto the external surface of a plastic container that is sufficiently durable to withstand the effect of pressurization of the container, retain an enhanced barrier to gases and vapors subsequent to said pressurization, and not affect the recyclability of the containers. Pressurized beverage containers currently comprise a very large market world-wide, and currently affordable plastics have sufficiently high permeation rates to limit the use of plastic containers in a number of the markets served.
Such pressurized containers include plastic bottles for both carbonated and noncarbonated beverages. Plastic bottles have been constructed from various polymers, predominant among them being polyethylene terephthalate (PET), particularly for carbonated beverages, but all of these polymers have exhibited various degrees of permeability to gases and vapors which have limited the shelf life of the beverages placed within them. For example, carbonated beverage bottles have a shelf-life which is limited by loss of CO2. (Shelf-life is typically defined as the time needed for a loss of seventeen percent of the initial carbonation of a beverage.) Because of the effect of surface to volume ratio, the rate of loss becomes greater as the size of the bottle is reduced. Small containers are needed for many market applications, and this severely limits the use of plastic bottles in such cases. Therefore, it is desirable to have a container with improved carbonation retention properties.
For non-carbonated beverages, similar limitations apply, again with increasing importance as the bottle size is reduced, on account of oxygen and/or water-vapor diffusion. It should be appreciated that diffusion means both ingress and egress (diffusion and infusion) to and from the bottle or container. The degree of impermeability (described herein as xe2x80x9cgas barrierxe2x80x9d) to CO2 diffusion and to the diffusion of oxygen, water vapor and other gases, grows in importance in conditions of high ambient temperature. An outer coating with high gas barrier can improve the quality of beverages packed in plastic bottles and increase the shelf life of such bottles, making small bottles a more feasible alternative, and this in turn presents many advantages in reduced distribution costs and a more flexible marketing mix.
Some polymers, for example PET, are also susceptible to stress cracking when they come in contact with bottle-conveyor lubricants used in bottle filling plants, or detergents, solvents and other materials. Such cracking is often described as xe2x80x9cenvironmental stress crackingxe2x80x9d and can limit the life of the bottle by causing leaks, which can cause damage to adjacent property. An impermeable outer surface for plastic bottles which surface resists stress-cracking inducing chemicals, prevents damage to adjacent property and will extend the shelf life of plastic bottles in some markets is highly desirable.
Another limitation to shelf life and beverage quality is often UV radiation which can affect the taste, color and other beverage properties. This is particularly important in conditions of prolonged sunshine. An outer coating with UV absorbing properties can improve the quality of such beverages and make plastic bottles much more usable under such conditions.
It is also desirable that plastic containers such as PET bottles be recyclable. Prior art barrier enhanced coatings, however, are often organic and relatively thick and therefore can contaminate a recycled plastic product. Organic coating materials incorporated into recycled plastic make unsuitable containers for beverage or food items because the beverage or food items can contact the organic coating material and become contaminated. In addition, relatively thick coatings form relatively large particles during recycling of plastic material and can damage the appearance and properties of a resulting recycled plastic product. In particular, relatively large coating particles in recycled plastic can make otherwise clear plastic hazy. Hazy plastic is often undesirable for containers such as beverage and food containers.
Finally, the cost of applying a coating to the outside of a bottle, which has a gas barrier which significantly increases the shelf-life of beverage container in that bottle, and/or which significantly reduces product spoilage of beverage container in that bottle, and/or which significantly reduces product spoilage due to UV radiation, and/or virtually eliminates environmental stress cracking, and/or provides a specific color, must not add significant cost to the basic package. This is a criterion which eliminates many processes for high gas barrier coatings, because plastic bottles are themselves a very low cost, mass produced article. Affordability implies in practice that the cost of the coating must add minimal or no increase to the cost of the whole package and in fact, the cost can be less.
A coating on the outside of plastic bottles must be capable of flexing. When bottles are used for pressurized containers, the coating preferably should be able to biaxially stretch whenever the plastic substrate stretches. In addition it is preferable that the coating be continuous over the majority of the container surface. Adhesion is particularly important in the case of carbonated beverages, since the CO2 within the bottle exerts some or all of its in-bottle pressure on the coating. This pressure can rise to above 6 bar, exerting considerable forces on the coating/plastic interface. The coating must also resist scuffing, normal handling, weathering (rain, sun climate, etc.), and the coating must maintain its gas barrier throughout the bottle""s useful life.
There are several plasma-enhanced processes which apply an external, inorganic coating to a range of articles, which in some cases includes bottles. Many of the processes are targeted to provide coating properties which are quite different, and far less onerous than high gas barrier bottle coatings. Such processes target, for example, abrasion resistance, where the coating continuity is not a major factor, since the coating can protect the microscopic interstices. Other processes target cosmetic or light-reflection properties and some processes have a pure handling protection role. Often the substrate does not flex nor stretch and the article itself is higher priced than plastic bottles so that cost is not a benefit of the design. In some cases, the substrate allows far higher coating temperatures than those allowed by PET, the most common plastic-bottle material. Such processes do not, in general, provide the coating continuity, adhesion, flexibility needed for high gas barrier coatings, nor do they provide a solution to the other problems relating to high gas barrier coatings, described above.
Prior art also exists for gas barrier processes for bottles, but the lack of commercially available, coated bottles for pressurized application is due to the fact that these processes lack the desirable attributes described above and fail to provide a coating with adequate adhesion, continuity and/or flexibility under high in-bottle pressure or a coating which avoids recycling problems, or the low cost necessary to make the coating affordable.
U.S. Pat. No. 5,565,248 to Plester and Ehrich describes a method for coating containers internally. However, external coatings require far greater adhesion than internal coatings, because in-bottle pressure acts against external coatings, and internal coatings are not subject to the same handling and/or abrasion in use. For these, and other reasons, coating bottles externally differs from coating them internally and the present invention is therefore substantially different.
For plastic containers such as PET bottles to be economically feasible containers for commercial products such as beverages and food, the bottles must be manufactured relatively inexpensively at a high speed and high volume. Accordingly, a process and system for coating plastic containers must be economical and capable of functioning at a high speed and high volume. Many prior art systems for coating objects with a gas barrier coating are batch processes or otherwise slow and inefficient.
Accordingly, there is a need for plastic containers which are coated with an effective gas barrier coating, can be efficiently recycled, and can be economically produced for use as containers for mass produced items such as beverages and food.
Accordingly, it is an object of the present invention to provide an outer coating or layer for a container such as a heat sensitive plastic bottle, and particularly for the non-refillable bottles used for carbonated beverages.
It is a further object of the present invention to provide a coating and a system and method for coating which can provide an external glass-like coating that is flexible, durable and possess sufficient adhesion to withstand the effects of pressurization, such as flexing and stretching of the container, and to withstand denting of the container, without significant loss of enhanced barrier properties.
An additional object of the present invention is to provide an externally coated container which will avoid environmental stress cracking such as when the container comes into contact with conveyor lubricants during filling and detergent, cleaners or solvents or similar substances during its life cycle. Such lubricants can include 409(trademark), Mean Green(trademark) or other commercially available cleansers or lubricants, etc.
Yet another object of the present invention is to provide a lighter container and a system and method for making the container whereby an amount of plastic utilized in making the container as compared to a conventional container can be reduced without adversely affecting or while improving the gas barrier effectiveness of the container.
It is another object of the present invention to provide a coating that comprises an inorganic oxide layer on the external surface of a plastic container, the inorganic oxide layer being further distinguished by being comprised of greater than or equal to 50 and up to but less than 100% SiOx (x=1.7 to 2.0).
Another object is to provide a coating which possesses sufficient adhesion to the external surface of the plastic container so that the barrier enhancement provided by the inorganic oxide layer is not substantially reduced upon pressurization of the container to a pressure between 1 and 100 psig.
A further object of the present invention is to provide a method for applying an inorganic layer as described above, the method resulting in a robust inorganic oxide layer that provides an effective level of barrier enhancement to the plastic container and does not result in significant physical distortion of the container.
It is a further object of the present invention to provide a system and method for manufacturing a container whereby the aesthetic appeal of the container will be enhanced by applying a colored inorganic layer that further contains visible-light absorbing species.
Yet another object of the present invention is to provide a coating for a container with UV absorbing capabilities.
Still another object of the present invention is to provide a container with a colored or clear coating which can easily be recycled without significant or abnormal complications to existing recycling systems.
Another object of the present invention is to provide a system and method for inexpensively manufacturing an externally coated container at high speed and high volume.
Yet another object of the present invention is to provide a method in which the thickness and composition of the applied coating on a container can be rapidly and easily determined and whereby process control and insurance of enhanced barrier performance can be obtained.
A further object of the present invention is to provide a method to determine the condition of the surface of a plastic container at least with regards to its suitability for applying glass-like coatings.
Another object of the present invention is to provide a high gas barrier which considerably increases the shelf life of the containers such as plastic bottles and to provide the containers with good transparency so as not to affect the appearance of a clear plastic bottle.
Still another object of the present invention is to provide a container with adequate durability and adhesion during working life, when the outer surface of the container is subjected to environmental conditions such as severe weather, rubbing, scuffing, or abrasions (for example, during transportation).
Also, another object of the present invention includes the ability to enable coating to heat sensitive plastic containers with coating materials, which can only be vaporized at very high temperatures without an acceptable increase in the plastic""s temperature and which must remain in many cases below 60xc2x0 C.
The foregoing and other objects of this invention are fulfilled by providing a coated plastic container comprising a plastic container body having an external surface and a coating on the external surface of the container body comprising an inorganic oxide and a glass-forming metal additive, wherein the coated plastic container, when containing a pressurized fluid sealed in the interior space of the container body at a pressure of 60 psig, possesses a gas barrier of at least 1.25xc3x97 the gas barrier of the container without the coating, when the container without the coating contains a pressurized fluid sealed in the interior space at a pressure of 60 psig. This invention also encompasses a method and system for making a coated plastic container possessing a gas barrier, a method fop recycling coated plastic containers, and a method and system for packaging beverages sealed in plastic containers including a gas barrier coating.
More particularly, the coated plastic container of this invention is made by depositing the coating on the exterior surface of the container body using vacuum vapor deposition, desirably plasma-enhanced vacuum vapor deposition. The resulting coating is desirably substantially homogeneous and amorphous and bonded either chemically or physically, or both, to the exterior surface of the container. As used herein, the term homogeneous means there is no substantial variation in atomic composition through the coating and the term amorphous means there is no substantial crystallinity in the coating as measured by standard x-ray diffraction techniques. In addition, the inorganic oxide and glass-forming metal additive are preferably present in the coating in concentrations which are substantially constant through the thickness of the coating. The resulting coating is therefore very durable.
Because of the high level of adhesion of the inorganic coating to the surface of the plastic container of the present invention, a continuous coating is not essential. In other words, even though the coating of the present invention may be non-continuous because of scratches or fractures therein, for example, the coating will continue to effectively adhere to the substrate such as an underlying plastic bottle. The present invention can therefore provide an effective gas barrier even if the surface is highly fractured. A high gas barrier of 1.25xc3x97 greater than the uncoated container can be obtained with the present invention and this barrier can even be 1.5xc3x97 or preferably 2xc3x97 greater than the uncoated container even when the coated container contains a pressurized fluid such as a carbonated beverage. In addition, the coated container of this invention has enhanced environmental stress crack resistance even when the container contains a pressurized fluid.
Furthermore, the coated container of the present invention can be made to have an equivalent gas barrier and reduced weight compared to a plastic container of similar surface area and volume and without said exterior inorganic coating.
The system of the present invention for making the coated plastic container comprises a vacuum cell, a container feeder, a conveyor and at least one source disposed in the vacuum cell for supplying a coating vapor. The vacuum cell is capable of maintaining a vacuum within the vacuum cell and the container feeder supplies plastic container bodies into and withdraws coated plastic containers out from the vacuum cell. The plastic container bodies each have an external surface and an interior surface defining an interior space. The conveyor conveys the plastic container bodies through the vacuum cell and the at least one source of coating vapor supplies coating vapor to the external surface of the container bodies as the container bodies are conveyed through the vacuum cell. The at least one source of coating vapor and the conveyor are structured and arranged within the vacuum cell such that the coating vapor from the at least one source deposits a thin coating on the external surface of the containers, the thin coating comprises an inorganic oxide and a glass forming metal additive and bonds to the external surface of the container bodies and the resulting coated plastic containers, when containing a pressurized fluid sealed in the interior space at a pressure of 60 psig, possess a gas barrier of at least 1.25xc3x97 the gas barrier of the containers without the coating, when the containers without the coating contain a pressurized fluid sealed in the interior space at a pressure of 60 psig. This invention also encompasses the corresponding method of making coated plastic containers.
Desirably, the system and method for making coated plastic containers of this invention are continuous and can operate at a high speed and high volume to economically mass produce the coated containers. More particularly, in the system and method for making a coated plastic container of this invention, while the vacuum cell maintains a vacuum within the vacuum cell, the container feeder continuously feeds the container bodies from outside the vacuum cell into the vacuum cell to the conveyor, the conveyor continuously conveys the container bodies through the vacuum cell passed the at least one source, and the container feeder continuously feeds the coated containers from the conveyors and withdraws the coated containers from the vacuum cell. Preferably, this system and method are automatic. The container feeder in the system and method of this invention is desirably a rotary feeder system capable of continuously and automatically feeding container bodies into and out of the vacuum cell at a high speed and a high volume while the vacuum cell maintains its vacuum. This high speed process allows the system and method of coating plastic containers to be placed in a high speed mass production process such as a beverage packaging line.
The coating vapor produced in the vacuum cell is desirably in the form of a plasma. A suitable device for producing the plasma is a cold cathode, also known as an electron gun. The plasma can optionally be energized with one or more antennas disposed in the vacuum cell using RF (radio frequency) or HF (high frequency) energy to form a high energy plasma.
Although a variety of vaporizable materials can be used to form the inorganic oxide coating in accordance with this invention as explained in more detail below, the inorganic oxide coating desirably comprises silica and glass forming metal additives such as zinc, copper, or magnesium.
The coating method and system of this invention also enables heat sensitive containers to be coated without significant temperature rise, and at all times maintaining a bottle temperature well below 60xc2x0 C. In addition, the coating method and system of this invention enables mixtures and layers of substances to be applied which can be chosen for their color, or UV-absorbing properties, or additional gas barrier properties. Further, the method and system of this invention enables coatings, such as silica, which are fully transparent and clear, and would therefore not affect the appearance of an otherwise clear bottle. The coating materials are inert and remain solid when the plastic bottle is melted for recycling.
Additional functionality can be incorporated into the inorganic coating of this invention by incorporating visible light absorbing species, rendering the plastic container cosmetically more appealing.
The method of this invention for producing recycled content plastic comprises the steps of providing a batch plastic, at least a portion of the batch plastic comprising coated plastic containers, and converting the batch of plastic to a form suitable for melt extrusion. Each coated plastic container comprises a container body having an external surface and a coating on the external surface comprising an inorganic oxide. The coated plastic containers can be made by the method described above and desirably have a very thin inorganic oxide coating. The coating preferably has a thickness from about 1 to about 100 nm.
Suitable methods of converting the batch of plastic to a form suitable for melt extrusion include grinding the batch plastic to produce flakes and melting the flakes to form a melt extrudable recycled plastic. Alternatively, the batch of plastic can be depolymerized and repolymerized to form a melt extrudable recycled plastic. The recycled plastic can be melt extruded into plastic articles such as recycled content plastic containers.
Because of the inert nature and thinness of the coatings of the present invention, the coated containers can be processed in any conventional recycling system without modification of the process. In addition, haziness in the resulting recycled articles is avoided in the present invention because the coating forms relatively small particles during recycling. Furthermore, the coating particles in the recycled plastic are acceptable for food contact and therefore do not adversely affect the recycling effort when ground or depolymerized in the recycling process.
The recycling method of the present invention provides for a method of recycling coated plastic which has results heretofore unattainable. In particular, separation of coated and uncoated plastics is unnecessary whereby modifications to existing recycling systems are unnecessary or whereby extra process steps (separating coated bottles from uncoated bottles) can be avoided. Moreover, it is possible to produce a transparent plastic from coated plastic while avoiding the above-noted problem of haziness in the final recycled product. While the present invention can be used in recycling many types of plastic, it is contemplated that this invention can be used with plastic articles, such as containers or bottles and more particularly, with plastic beverage bottles. Bottle-to-bottle recycling remains unaffected with the present invention. The coating of the present invention does not interfere with the downstream injection molding or blow molding of recycled plastic.
The method of packaging a beverage in accordance with this invention comprises the steps of providing a coated plastic container, filling the plastic container with the beverage and sealing the plastic container after the step of filling. The coated plastic container comprises a plastic container body having an external surface and a coating on the external surface comprising an inorganic oxide. This coating provides a gas barrier and desirably is the coating described hereinabove. The gas barrier coating inhibits the flow of gas into and out of the container. For example, the gas barrier coating can protect the beverage from the flow of oxygen into the container from the outside or can inhibit the flow of carbon dioxide out of the beverage container. The method and system of packaging a beverage according to this invention is particularly useful in producing carbonated beverages. Such a method further comprises the steps of carbonating the beverage before the filling step and then sealing the beverage under pressure in the coated container. The resulting carbonated beverage has a longer shelf life because the coating on the container better holds the carbon dioxide within the container.
The method and system of packaging a beverage according to this invention is desirably a high speed, high volume process wherein the coated plastic containers are continuously provided, the plurality of plastic containers are continuously filled with the beverage, and the filled containers are continuously sealed. Accordingly, the method and system for packaging a beverage can form a single continuous processing line including the plastic container body production, the process for coating the plastic container, and the steps of filling the plastic containers with a beverage and sealing the plastic container after the step of filling, although such a single continuous processing line is not necessary.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.